The present invention relates to a blasting medium and a blasting method. Particularly, it relates to a blasting medium and a blasting method, whereby fine processing is possible without bringing about excessive damages or functional deterioration to the object to be treated, and even when the medium remains on the object to be treated, it can readily be removed.
In recent years, along with the progress of electronic engineering, integration and accelerating of computing functions have been advanced with respect to electronic equipments such as computers. Multilayer interconnection boards are not exception, and various multilayer interconnection boards have been developed which make high density wiring or high density packaging possible.
As a method for forming such multilayer interconnection boards, a production method by a build up method is known wherein via holes are formed to electrically connect an upper layer wiring pattern and a lower layer wiring pattern. In this method for production of multilayer interconnection boards by a build up method, it takes time for forming via holes, and work efficiency is poor. In order to overcome such a drawback, a system has been proposed wherein an interlayer insulation layer made of a resin composition is selectively irradiated with a high power laser beam such as a carbon dioxide gas laser beam or an excimer laser beam to thermally decompose, dissipate and remove optional portions of the interlayer insulation layer by the laser beam thereby to form via holes.
FIG. 1 illustrates a multilayer interconnection board having via holes formed in an interlayer insulation layer by means of a laser beam according to this system. Referring to FIG. 1, a wiring pattern 3 of from about 1 to 200 xcexcm is formed on a substrate 1. Then, an interlayer insulating layer 5 is formed thereon. Thereafter, optional positions of the interlayer insulating layer 5 are treated for thermal decomposition, etc. by a laser beam to form via holes 7.
By the introduction of such a system, formation of via holes 7 can be completed in a relatively short period of time, and the work efficiency has been substantially improved. However, part of the interlayer insulation layer which has been thermally decomposed, dissipated and removed is likely to re-attach or deposit on the inner walls or peripheral portions of the via holes 7 to form smears 9. Such smears 9 bring about drawbacks such that they cause failure in electrical conduction to a lower layer wiring pattern during treating treatment, or they may change the thickness of the interlayer insulation layer 5.
To overcome such drawbacks, it has been proposed to immerse the substrate 1 having via holes 7 formed, in a smear-removing liquid. However, with a conventional smear-removing liquid, it has been difficult to completely remove smears 9, and it has had a problem that the liquid tends to deteriorate the interlayer insulation layer 5.
Further, if an interlayer insulation layer 5 made of an epoxy resin composition containing inorganic fibers of e.g. glass, is irradiated with a high power laser beam, only the resin portion tends to be removed, and the inorganic fibers tend to be exposed in via holes 7, whereby it has been difficult to form an electrically conductive layer. Further, such exposed fibers can hardly be removable by means of the smear-removing liquid, whereby it has been extremely difficult to form a good conductive layer.
On the other hand, smear treatment by means of a blast material is also known. As is different from the case where a smear-removing liquid is employed, this treatment is very effective to cleanly remove smears 9 and exposed inorganic fibers and to form via holes 7 with excellent configuration.
However, especially when via holes 7 of not larger than 100 xcexcm are to be treated by a blasting method, with a conventional blasting material such as glass beads, alumina, silica, silicon carbide or zirconium oxide, such a blasting material is likely to collect and clog via holes 7 and can hardly be completely removed even by a subsequent washing process, thus leading to failure in forming a good conductive layer.
The present invention has been made in view of such problems of the prior art, and it is an object of the present invention to provide a water-soluble blasting medium and a blasting method, whereby fine processing is possible without bringing about excessive damages or functional deterioration to an object to be treated, and even if the medium remains on the object to be treated, it can readily be removed.
That is, the present invention provides a blasting medium which has an average grain size of at most 20 xcexcm and contains at least 90 mass % of a water-soluble inorganic salt, wherein the content of grains having grain sizes of at least 50 xcexcm is at most 5 mass %.
The average grain size is represented by a numerical value of the average grain size based on the volume measured by means of a laser diffraction scattering grain size distribution measuring apparatus. Hereinafter, the average grain size means a value measured by this method, unless otherwise specified. Further, with respect to the representation by %, for example, when a content of grains of at least 44 xcexcm is represented by X %, such representation means a mass proportion (X %) of a sample remaining on a sieve when a sample of 50 g is put on a sieve of a metal net with openings of 44 xcexcm and sieved for 10 minutes by a ro-tap shaker.
For example, when smear-removing treatment after formation of via holes in a multilayer interconnection board, is carried out by a blasting method, the blasting medium to be used, is required to be able to cleanly remove smears and expose inorganic fibers and not to remain in the via holes.
The blasting medium of the present invention has an average grain size of at most 20 xcexcm, and the content of grains having grains sizes of at least 50 xcexcm is at most 5mass % (hereinafter referred to simply as %). Namely, it is of a size particularly suitable for smear-removing treatment of via holes of not larger than 100 xcexcm.
Further, the blasting medium of the present invention contains at least 90% of a water-soluble inorganic salt, whereby even when a plurality of grains enter into via holes to clog them, they can be cleanly removed by washing with water.
Therefore, the blasting medium of the present invention is very effective for fine processing such as smear-removing treatment of via holes. It is thereby possible to form via holes having excellent configuration, whereby close contact between an interlayer insulation layer and a plated conductive layer is secured, and it is possible to produce a multilayer interconnection board having high reliability.
Further, the blasting medium of the present invention is also very effective for an operation of forming a pattern such as a partition wall formation in a plasma display or for an operation of removing resin burrs in resin-sealed type semiconductor devices, in the similar fine processing fields.
Thus, the blasting medium of the present invention is excellent in the work efficiency in the fine processing fields, and even when it is remained in the object to be treated, it can readily be removed.
Further, in the present invention, the above-mentioned inorganic salt is preferably sodium hydrogencarbonate and/or potassium hydrogencarbonate.
Sodium hydrogencarbonate and/or potassium hydrogencarbonate (hereinafter referred to as sodium hydrogencarbonate or the like) is suitable for blasting treatment and has a proper hardness, so that it has little possibility of giving excessive damages to the substrate or the like. Here, sodium hydrogencarbonate has no moisture absorption property and is easy to handle. However, if sodium is desired not to remain, it is preferred to use potassium hydrogencarbonate.
In the present invention, the blasting medium preferably contains an anti-caking agent having an average particle size of at most 20 xcexcm.
The blasting medium of the present invention may undergo agglomeration when stored for a long period of time, because sodium hydrogencarbonate or the like having a small particle size, is used. Therefore, it is preferred to incorporate an anti-caking agent, whereby the blasting medium will be free from agglomeration, and it is possible to improve the work efficiency in blasting.
The anti-caking agent is preferably silica. The silica may, for example, be fumed silica or white carbon.
Further, the anti-caking agent is preferably hydrophilic silica. If the anti-caking agent is hydrophilic, the operation will be facilitated for removal of the blasting medium or for treatment of the removed waste material.
Further, the present invention provides a blasting method which comprises blasting an object to be treated, with the above-blasting medium using a fluid as a blast carrier.
Here, the fluid is meant for a gas, a liquid or the like.